Tin plating is known. Tin has many desirable attributes as a plated finish including solderability, lubricity, good electrical conductivity, and corrosion resistance. Tin is a silver-colored, ductile metal whose major application is to impart solderability to otherwise unsolderable base metals. Tin has generally good covering characteristics over a wide range of shapes. While tin does not tarnish easily, tin is a soft, ductile finish that can scratch or mar easily. Tin is reported to be non-toxic and non-carcinogenic and thus is approved for food container and food contact applications.
Unfortunately, tin plating suffers from a significant drawback, the undesirable formation of spontaneous latent whiskers. Whiskering involves the formation of thin, needle-like crystals after plating. Whiskers typically form from a few weeks to several years after plating. Common whiskers may measure up to 2.5 μm in diameter, and can grow to a length of 10 mm. Conditions that tend to promote the growth of whiskers are compressive stresses and uniform temperatures for long periods of time. In some applications using tin plating, whiskers are not functionally noticed and therefore harmless. However, in other applications, such as closely spaced electronic circuits, whiskers undermine the operational function of devices employing a tin plating product. For example, in electronic circuitry, whiskers are capable of carrying sufficient current at low voltages to cause short circuits or a corona discharge. Whiskers can have a current carrying capacity of as high as 10 mA.
Attempts to eliminate the possibility of tin whiskering involve alloying the deposit of tin with lead. It is desirable to include more than 3% by weight lead in a tin-lead alloy to insure no latent whiskering. Many applications even call for increased lead contents towards the eutectic, thereby depressing the melting point of the alloy. Due to disposal, environmental, and health concerns, deposits that contain lead, as well as the general use of lead, are no longer desirable.